The present invention relates generally to steam turbines and, more particularly, to the design of a blade stage structure of an full-arc admission throttle governing type steam turbine.
It is said that an overall improvement in the internal efficiency of a steam turbine could save a considerable amount of money in fuel costs if the internal efficiency could be improved by just 1%, as in the case of a 100 MW power plant. Therefore, if the internal efficiency could be improved by 1 to 2%, the power plant would pay for the additional cost of the hardware.
The prior art has classified steam plants having a constant input steam pressure from its running standpoint into two types, a plant that is run in a rated load operation and a plant that is run relatively frequently under a partial load, with an output of no more than the rated point. Since the former category is run at the rated operation, the full-arc admission throttle control type or "full-arc admission" steam turbine, has no control stage to lower its efficiency and is therefore more efficient and advantageous. On the other hand, the latter category is frequently run under partial load, and has a Curtis or Rateau stage as the control stage.
These control stages are more advantageous for partial loads when using a nozzle control as shown in FIG. 3.
An example of the prior art will be described with reference to FIG. 4, which is a diagram showing a partial structure of the full-arc admission steam turbine, and to FIG. 5 which is the pressure-output diagram of the turbine. In the full-arc admission steam turbine, the steam under a pressure P.sub.0 is introduced through a first steam regulating valve 3 at a flow rate G.sub.0 into a chamber at an introduction pressure P'.sub.1 so that it generates power by turning a rotor 9 while expanding through a blade stage group 10. The output N.sub.P is deduced from the following formula: EQU N.sub.P =G.sub.0 .times..DELTA.i.sub.0 /0.86.times..sup..eta. i.sub.(N),
wherein:
N.sub.P is the output; G.sub.0 is the amount of steam; PA1 .DELTA.i.sub.o is the adiabatic heat drop (i.e., the enthalpy difference); and .sup..eta. i.sub.(N) is the internal efficiency of the turbine.
The relationship between the pressure and the output is generally proportional as plotted by the curve P.sub.1 in the pressure-output diagram of FIG. 5.
FIG. 3 is a comparison diagram of the internal efficiency of the present invention as compared to the throttle and nozzle prior art. The diagram represents the relationship between the internal efficiency and the output. This is accomplished by plotting the internal efficiency ratio .eta. and the output ratio N on the ordinate and abscissa, respectively, in percentages. In this diagram, the curve a represents the throttle governing type steam turbine of the present invention; the curve b represents the throttle governing type steam turbine of the prior art; and the curve c represents the nozzle cut-off governing type steam turbine.
It is further apparent from this diagram that the efficiency of the full-arc admission steam turbine of the prior art drops at 70% of output, as represented at P, the intersection between the curve b and the abscissa scale of 70%. Although the internal efficiency of the nozzle cut-off governing type steam turbine drops to a point q at most, the former turbine is less advantageous than the latter turbine. This is because the steam flow rate is controlled to reduce the output by throttling the steam regulating valve so that the internal efficiency drops due to the throttle loss of the valve.
As described above, the full-arc admission steam turbine of the prior art has an excellent efficiency in the rated load operation but is deficient, as shown by the drop in efficiency, for a partial load. The present invention overcomes this problem and provides an full-arc admission steam turbine the internal efficiency of which drops only marginally even under partial load.